Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
In a turbocharger it is often desirable to control the flow of exhaust gas to the turbine to improve the efficiency or operational range of the turbocharger. Variable geometry turbochargers have been configured to address this need. A type of variable geometry turbocharger is one having a variable exhaust nozzle, referred to as a variable nozzle turbocharger. Different configurations of variable nozzles have been employed in variable nozzle turbochargers to control the exhaust gas flow. One approach taken to achieve exhaust gas flow control in such variable nozzle turbochargers involves the use of multiple pivoting vanes that are positioned annularly around the turbine inlet. The pivoting vanes are commonly controlled by a unison ring, that is movably disposed within the turbocharger, to alter the throat area of the passages between the vanes, thereby functioning to control the exhaust gas flow into the turbine.
In such variable nozzle turbochargers, the unison ring is configured to rotate in response to a controlled actuator movement, which rotational movement causes the vanes to pivot in a manner opening or closing the flow path of exhaust gas to the turbine wheel. In order for the variable nozzle turbocharger to function properly, it is important that the desired rotational movement of the unison ring not be restricted or impaired. It is known that in such variable nozzle turbochargers, impaired unison ring movement can result from the build up of moisture within the turbocharger, which is known to cause unwanted corrosion between the unison ring and adjacent turbocharger surfaces. This corrosion can operate to impair and restrict proper unison ring movement with the turbocharger.
It is, therefore, desirable that a unison ring assembly be constructed, for use with a variable nozzle turbocharger, in a manner that facilitates desired unison ring movement in a manner that minimizes or eliminates altogether potential impairments to movement caused, e.g., by moisture build up and corrosion.